Multiple cpp files

I've read through the articles and previous forum questions on the subject, but couldn't find what I wanted, so...

I've been creating a program for a while now using just one .h and one .cpp, but it's starting to get big and messy with all the different classes I'm using, so I'd like to cut the class definitions into separate .cpp files. I tried doing that for just one class and immediately got "unresolved external" errors regarding the class I'd just moved to a new .cpp. Here's the code (I've removed the actual definitions to save space).